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Recent advances in bulk heterojunction (BHJ) polymer solar cell (PSC) performance have resulted from compressing the band gap to enhance the short-circuit current density (JSC) while lowering the highest occupied molecular orbital to increase the open-circuit voltage (VOC) and consequently enhance the power conversion efficiencies (PCEs). However, PCEs of PSCs are still constrained by a low JSC, small VOC, and low fill factor (FF). In this study, we report 10.12% PCE from single-junction PSCs based on a novel two-dimensional (2D) conjugated copolymer. By introduction of conjugated 5-alkylthiophene-2-yl side chains to substitute nonconjugated alkoxy side chains in one-dimensional (1D) poly[4,8-bis[(2-ethylhexyl)oxybenzo1,2-b:4,5-b'dithiophene-2,6-diyl]3-fluoro-2-[(2-ethylhexyl)carbonylthieno3,4-bthiophenediyl]] (PTB7), a novel 2D donor-acceptor low-band-gap conjugated copolymer, poly[4,8-bis[(5-ethylhexyl)thienylbenzo1,2-b;3,3-bdithiophene-2,6-diyl]3-fluoro-2-[(2-ethylhexyl)carbonylthieno3,4-bthiophenediyl]] (PTB7-DT), is developed. 2D PTB7-DT is further systematically investigated by absorption spectroscopy, cyclic voltammetry, charge carrier mobility measurement, thin film morphology, and wide-angle X-ray diffraction and compared with 1D PTB7. In comparison with 1D PTB7, 2D PTB7-DT possesses a narrower band gap, tighter π-π stacking, and higher charge carrier mobility. These results are consistent with the observation from first-principle calculations. Consequently, the single-junction PSCs based on 2D PTB7-DT exhibit a PCE of 10.12% with a high JSC, larger VOC, and high FF in comparison with the PSCs based on 1D PTB7.
Liu et al. (Wed,) studied this question.